This invention relates to a method of forming a coating on a substrate by depositing a solution of a resin containing Si--H groups and a solvent in a manner in which solvent remains in the coating followed by exposing the coating to a basic catalyst and water and then evaporating the solvent from the coating. The method of the invention is particularly useful for applying low dielectric constant coatings on electronic devices.
Thin film dielectric coatings on electronic devices are known in the art. For instance, Haluska et al. in U.S. Pat. Nos. 4,749,631 and 4,756,977, which are incorporated herein by reference, disclose silica based coatings produced by applying solutions of silicon alkoxides or hydrogen silsesquioxane, respectively, to substrates and then heating the coated substrates to temperatures of 200-1000.degree. C. The dielectric constant of these coatings, however, are often too high for certain electronic devices and circuits.
Haluska et al. in U.S. Patent Nos. 4,847,162 and 4,842,888 also teach the formation of nitrided silica coatings by heating hydrogen silsesquioxane resin and silicate esters, respectively, to a temperatures of between 200 and 1000.degree. C. in the presence of ammonia. These references teach the use of anhydrous ammonia so that the resulting coating has about 1 to 2 % by weight nitrogen incorporated therein.
Glasser et al. in the Journal of Non-Crystalline Solids, 64 (1984) pp. 209-221 teach the formation of ceramic coatings by heating tetraethoxysilane in the presence of ammonia. As with Haluska '162 above, however, this reference also teaches that the ammonia should be anhydrous and that the resultant silica coatings are nitrided.
Jada in U.S. Pat. No. 4,636,440 discloses a method of reducing the drying time for a sol-gel coated substrate comprising exposing the substrate to aqueous quaternary ammonium hydroxide and/or alkanol amine compounds. The methods of this reference, however, are different than those disclosed herein in that Jada requires the coating to be dried prior to heating. Moreover, Jada is specifically limited to hydrolyzed or partially hydrolyzed silicon alkoxides and fails to teach the utility of the process on coatings having Si--H bonds.
Haluska et al. in U.S. Pat. No 5,262,201 and Baney et al. In U.S. Pat. No. 5,116,637 teach the use of basic catalysts to lower the temperature necessary for the conversion of various preceramic materials, including hydrogen silsesquioxane, to ceramic coatings. These references, however, teach the removal of solvent before the coating is exposed to the basic catalysts.
Camilletti et al. in U.S. Pat. No. 5,547,703 teach a method for forming low dielectric constant Si--O containing coatings on substrates comprising heating a hydrogen silsesquioxane resin successively under wet ammonia, dry ammonia and oxygen. The resultant coatings have dielectric constants as low as 2.42 at 1 MHz. Again, however, this reference teaches the removal of solvent before converting the coating to a ceramic and the dielectric constant of the resultant coatings are not as low as those disclosed herein.
Ballance et al. in U.S. Pat. No. 5,523,163 teach a method for forming Si--O containing coatings on substrates comprising heating a hydrogen silsesquioxane resin to convert it to a Si--O containing ceramic coating and then exposing the coating to an annealing atmosphere containing hydrogen gas. The resultant coatings have dielectric constants as low as 2.773. Again, however, this reference teaches the removal of solvent before converting the coating to a ceramic and the dielectric constant of the resultant coatings are not as low as those disclosed herein.
Syktich et al in U.S. Pat. No. 5,618,878 teach coating compositions containing hydrogen silsesquioxane resin dissolved in saturated alkyl hydrocarbons useful for forming thick ceramic coatings. The alkyl hydrocarbons disclosed are those up to dodecane. The reference, however, fails to teach exposure of coated substrates to basic catalysts before solvent removal.
The present inventors have now discovered that by exposing a Si--H containing resin to basic catalysts before complete solvent removal, porous network coatings with low dielectric constants can be produced.